Eos,Vol. 80, No. 33, August 17,1999 any substantial difference in the predicted warming is a fairly shortsighted view. While it is true that a few percent reduction won't b e enough, it is a start, and that's what is impor tant from the scientific point of view.This is just the beauty of Kyoto. It challenges us to make a modest start. By 2008-2012, when the Kyoto agreements must b e met, we will have a much firmer idea of any possible threats. If by then (or even before) we have learned that the warming will not b e large or at least not troublesome, we are free to moderate our car b o n restrictions. But if we have found that there is a significant problem, we will already b e well started and positioned e c o n o m i c a l l y and scientifically to a c c e l e r a t e our reductions in c a r b o n emission. There is a further reason for keeping a t m o s p h e r i c c a r b o n to a minimum; even though the predicted rate of temperature rise with increasing c a r b o n c o n c e n t r a t i o n slows, the c a r b o n is still there and will take a very long time to go away. Thus, in the event we triple c a r b o n dioxide in the atmos phere, we will b e stuck with its effects for m o r e than a hundred years.
Reply Does the "balance of evidence suggest a dis cernible human influence on climate?" Until properly defined, this main conclusion of the IPCC [1996,p.5] is not meaningful.This ambiguous phrase does not identify (using Keller's words) "a small but growing [anthro pogenic] signal in a noisy chaotic climate." Nor can it b e used "to project future warming even with admittedly imperfect climate models." In fact, there is an explicit denial of such a link in the IPCC report itself [IPCC, 1996, p. 4 3 4 ] : "To date, pattern-based studies have not b e e n able to quantify the magnitude of a greenhouse gas or aerosol effect on climate." Yet when politicians at the 1996 Geneva climate convention (COP-2) asserted such a link and thereby misused the IPCC phrase to claim substantial future warming and im pending climate disasters, the IPCC lead ership did not protest but simply a c q u i e s c e d . The basis for the IPCC phrase is presented in Figure 1 (which reproduces figure 8.10(b) of the IPCC report). It plots the correlation coefficient between observed and calculated geographic warming patterns. It also shows a straight line labeled "50-Year linear trend" to suggest a steady increase of this coefficient since 1940, presumably b e c a u s e of a growing signal from human activities that gradually overcomes the "noise" of natural climate fluc tuations. This graph is derived from a research paper [Santer et al., 1995] that had not yet b e e n published at the time the IPCC draft was finalized. In the research paper, the s a m e graph appears, but with a multitude of trend
For more reading, NASAs Jim Hansen has put his recent debate with Pat Michaels on the Web (http://www.giss.nasa.gov/edu/ gwdebate/gwdebate.pdf). Hansen took a slightly different approach, which gives a different slant on the issues involved in this complicated problem.
Author CF Keller Institute of Geophysics and Planetary Physics, Los Alamos National Laboratory, N. Mex., USA
Intergovernmental Panel o n Climate Change (IPCC),Working Group \, Climate Change 1995: The Science of Climate Change, J.T.Houghton et al., eds., 572 pp., Cambridge University Press, New York, 1996. J o n e s , RD.,K. R. Briffa,T. PBarnett, and S. FB.Tett, High-resolution p a l a e o c l i m a t i c records for the last millennium, The Holocene, 5,455471,1998. Karl, R.T., and R. W Knight, Secular trends of precipi tation amount, frequency, and intensity in the United States, \998, Bull. Am. Meteorol. Soc, 79, 231-241,1998. Mann, M. E., R. S. Bradley, and M. K. Hughes, Globals c a l e temperature patterns and climate forcing over the past six centuries, Nature, 392,779,1998. Revelle, R., a n d H. E. Suess, Carbon dioxide e x c h a n g e b e t w e e n a t m o s p h e r e and o c e a n and the question of an increase of atmospheric C 0 during the past d e c a d e s , TellusJX, 19-20,1957. Rind, D., J. Lean, a n d R. Healy,Simulated timed e p e n d e n t climate response to solar radiative forcing s i n c e 1 6 0 0 , i Geophys. Res., 104,19731990,1999. Shindell, D., D. Rind, N. Balachandran, J. Lean, and PLonergan, Solar c y c l e variability, ozone, and cli mate, Science, 284,305,1999. T h o m p s o n , L. G , et al, A 25,000-year tropical climate history from Bolivian i c e cores, Science, 282,1858, 1998. T i m m e r m a n n , A., J. Oberhuber, A. Bacher, M. Esch, M. Lativ, and E. Roeckner, Increased El Nino frequency in a climate m o d e l forced by future g r e e n h o u s e warming, Nature, 398,694-696,1999. 2
References Bengtsson, L., E. R o e c k n e r , a n d M. S t e n d e l , W h y is global w a r m i n g p r o c e e d i n g m u c h slower than e x p e c t e d ? , J. Geophys. Res., 104,3865-3876,1999. Cess, R. D., et al., C o m p a r i s o n of the s e a s o n a l c h a n g e in cloud-radiative forcing from a t m o s p h e r i c general c i r c u l a t i o n m o d e l s a n d satellite observations, J. Geophys. Res., 102, 16,593-16,603,1997. G o o d r i d g e , J . D., C o m m e n t s o n regional simulations of g r e e n h o u s e warming including natural variability,Bull. Am. Meteorol. Soc, 77, 1588-1589,1996.
lines, including zero and negative trends ( s e e Figure 1), depending on an arbitrary c h o i c e of time interval. [It is ironic that during the time interval 1910 to 1940 when global tem peratures were rising sharply, the correlation coefficient is seen to decrease.]
Scientific Disputes The paleorecord There are abundant records showing large and rapid temperature variations in the past, exceeding the predictions of most climate models. Until recently all these were based on proxy records. Now we have measure ments in Greenland i c e cores, going back s o m e 100,000 years, using thermometers [Dahl-Jensen et al., 1998] .The existence of the Medieval Climate Optimum is clearly evident, as is the Little Ice Age and the subsequent sudden warming between 1850 and 1940.The authors remark explicitly that they observed cooling between 1940 and 1995; this is con trary to expectations from climate models, which calculate an especially strong warming at high latitudes. Surface versus satellite data Only satellite data c a n furnish a truly global temperature record; few would agree with Keller, however, that the observed trends for satellite and surface data are similar.The latter show a slight positive trend; satellites (even after corrections) and the independent bal loon data do not. A n u m b e r of authors have tried to explain the c a u s e of the discrepancy Keller suggests the effects of ozone depletion during the period 1992 to 1997. But total
ozone has actually increased since 1993 [WMO, 1999,p.4.22] in spite of the continuing rise in stratospheric chlorine. [In my view, surface data are contaminated in various ways, including by the urban heat island (UHI) effect [Goodridge, 1996]. Earlier, Karl and J o n e s [1989] demonstrated the UHI effect using U.S. data.] The real issue, of course, is not the rather minor divergence a m o n g the data, but the lack of agreement between observations and model results. Models predict a warming trend in the middle and upper troposphere of about 0.5°C per d e c a d e , double the nearsurface value, while satellite and balloon observations still show a small cooling trend. Strenuous efforts are being made to validate the models against observations. According to Hansen et al. [ 1 9 9 8 ] , t h e uncertainties in radiative forcing are so large as to make it impossible to predict climate changes. Other researchers have m a d e use of these s a m e uncertainties (in the forcings from aerosols, o z o n e changes, and solar variations) to c o b ble together an agreement with observa tions, using this large number of adjustable parameters. Impacts of climate change Patrick Michaels has dealt adequately with the issue of changing precipitation patterns in the United States (which, after all, o c c u pies only 1% of the Earth's surface).The IPCC report [1996, p. 155] cites a decreasing pre cipitation trend after about 1950 for most of the globe.There is no e v i d e n c e that severe storms or hurricanes should increase if glob al temperatures were to rise [Henderson-Sell ers et al,1998].
Eos,Vol. 80, No. 33, August 17,1999 0.6
the opposite conclusion: Warming is bene ficial and increases Gross Domestic Product [Mendelsohn and Neumann, 1999]. Limitations of s p a c e do not permit a response to other issues. For further reading, I recommend my b o o k Hot Talk, Cold Science: Global Warming's Unfinished Debate, now in its s e c o n d edition. Our Web site carries more detailed information and carries a weekly bul letin with relevant news items.
CO2 + sulphate aerosols a
0.4
o ,2 t
0.2
u
o.o
a \-> 5 0 - y e a r trend ( 1 9 4 4 - 9 3 )
Z
4()-ycar trend ( 1 9 5 4 - 9 3 )
-0.2
Author
3 0 - y e a r trend ( 1 9 6 4 - 9 3 ) 2 0 - y e a r trend ( 1 9 7 4 - 9 3 ) 10-year trend ( 1 9 8 4 - 9 3 )
-0.4-J
-0.6 + 1900
1
1
•
1
1
1
1920
1
1
i
i
1940
i
1
1960
i
i
i
1
i
i
i
1980
2000
Year
S. Fred Singer The S c i e n c e & Environmental Policy Project, Fairfax,Va., USA; E-mail:
[email protected]; Web site: http://www.sepp.org
References
0.6
CO2 + sulphate aerosols 0.4
o 0.2
O 0.0 U e 4—*
-0.2
50-year linear trend
Oh
-0.4 -0.61900
1920
1960
1940
1980
2000
Year Fig. 1. Pattern correlation between observed and calculated geographic temperature distributions, as given by a correlation coefficient R(f). The top graph is taken from the original research paper by Santer et al. [1995]. As can be seen, R(t) is rather small and shows strong variability. It decreas es during the period of rapid global warming (before 1940) and does not increase during the past 25 years when atmospheric C0 levels rose sharply. The bottom graph is taken from the 1996 IPCC report (Figure 8.10b). It shows only an increasing 50-year trend line; the zero and neg ative trend lines were edited out. 2
A statistical analysis of El Nino events shows a decrease during the early part of the centu ry while temperatures increased. A statistical analysis of sea-level data [Singer, 1999b] also shows an inverse correlation with tempera ture changes. Evidently, increased warming causes more o c e a n evaporation, leading to more precipitation and ice accumulation in
Antarctica (thereby lowering sea level), which dominates over sea-level rise from the melting of mountain glaciers and the thermal expan sion of o c e a n water. Almost everyone agrees that a global warm ing, and especially the increase in carbon dioxide, benefits agriculture. Economists have re-evaluated the IPCC studies and arrived at
Dahl-Jensen, D., K. Mosegaard, N. Gundestrup, G. D. Clow, S. J. J o h n s e n , A.W. Hansen, a n d N. Balling, Past temperatures directly from the Greenland ice sheet,Science, 282,268-279,1998. Goodridge,J. D., C o m m e n t s on regional simulations of greenhouse warming including natural variabil ity, Bull.Amer. Meteorol. Soc, 11,3-4,1996. Hansen, J. E., M. Sato, A. Lacis, R. Ruedy, I.Tegen, and E. Matthews, Climate forcings in the Industrial era, Proc. Natl. Acad. Sci. USA, 95,12753-12758,1998. Henderson-Sellers, A., H. Zhang, G. Berz, K. Emanuel,W. Gray, C. Landsea, G. Holland, J . Lighthill, S.-L. S h i e h , RWebster, a n d K. McGuffie, Tropical c y c l o n e s a n d global c l i m a t e c h a n g e : A post-IPCC a s s e s s m e n t , B u l l . Am. Meteorol. Soc, 79,19-38,1998. IPCC WG-I, Climate change 1995: The science of climate change, (J.T. Houghton, L. G. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg, a n d K. Maskell, e d s . ) , C a m b r i d g e University Press, Cambridge, UK, 1996. Karl,T. R., a n d PD. J o n e s , Urban b i a s in area-aver a g e d s u r f a c e air temperature trends, Bull. Am. Meteorol. Soc, 7 0 , 2 6 5 - 2 7 0 , 1 9 8 9 . M e n d e l s o h n , R., a n d J. E. N e u m a n n ( e d s . ) , The impact of climate change on the United States economy, C a m b r i d g e University Press, Cambridge, UK, 1999. Santer, B. D., K. E. Taylor, T. M. L. Wigley, J. E. Penner, PD. J o n e s , a n d U. C uba sc h,Towa rds the d e t e c t i o n a n d attribution of an a n t h r o p o g e n i c effect o n climate, Climate Dynamics, 12,79-100,1995. Singer, S.E, H u m a n contribution to c l i m a t e c h a n g e r e m a i n s q u e s t i o n a b l e , Eos, Transactions, Ameri can Geophysical Union, 80,186-187,1999a. Singer,S. F.,Hot talk, cold science: Global warming's unfinished debate, T h e I n d e p e n d e n t Institute, O a k l a n d , Calif., 57 pp., ( S e c o n d e d i t i o n ) , 1999b. WMO [World M e t e o r o l o g i c a l Organization] Scien tific assessment of ozone depletion: 1998, Report No. 44, G e n e v a , February 1999.
